In a four year longitudinal study, the impact of menarche and its timing is being evaluated for adolescent girls. Predictions regarding adaptive and maladaptive adolescent development derive from a model of girls' biopsychosocial development, viz, that a) the effects of the biological event of menarche and its timing in girls' lives are b) mediated by variables such as physical status, development of secondary sex characteristics, preparation for menstruation, ego functioning, sense of being developmentally on- or off-time, and are evident in c) three classes of biopsychosocial functioning related to physical development and body experience, personality, and interpersonal behavior. In a cohort-sequential design, two cohorts of girls, all premenarcheal at the beginning of the study, will be followed from sixth through the ninth grade, being tested in the fall and spring of each year. As girls reach menarche during the course of the study, they will be classified as early, average or late maturers depending on their age and grade at the onset of menstruation. The study will be completed with a total sample of 200 girls, all followed for four years. Multivariate analyses of variance and regression analyses will be used to answer questions related to a) psychosocial impact of menarche, b) variations in the impact of menarche for early, average, and late maturers, c) variables mediating the impact of menarche, d) stability of developmental change following menarche and e) relation of menarche and its timing to adaptive and maladaptive adolescent functioning. Results will inform programs oriented both to the prevention and treatment of health related problems which often begin in adolescence, e.g., depression and eating/weight disorders.